The invention relates to a magnetic resonance method for forming a fast dynamic image from a plurality of signals acquired by an array of multiple sensors according to the preamble of claim 1. The invention also relates to a magnetic resonance imaging apparatus for obtaining a fast dynamic image according to the preamble of claim 17 and to a computer program product according to the preamble of claim 18.
In magnetic resonance imaging there is a general tendency towards obtaining acceptable images within shorter periods of time. For this reason the sensitivity encoding method called xe2x80x9cSENSExe2x80x9d has recently been developed by the Institute of Biomedical Engineering and Medical Informations, University and ETH Zxc3xcrnich, Switzerland. The SENSE method is based on an algorithm which acts directly on the image as detected by the coils of the magnetic resonance apparatus and in which subsequent encoding steps can be skipped and hence an acceleration of the signal acquisition for imaging by a factor of from two to three can be obtained. Crucial for the SENSE method is the knowledge of the sensitivity of the coils which are arranged in so-called sensitivity maps. In order to accelerate this method there are proposals to use raw sensitivity maps which can be obtained through division by either the xe2x80x9csum-of-squaresxe2x80x9d of the single coil references or by an optional body coil reference (see e.g. K. Pruessmann et. al. in Proc. ISMRM, 1998, abstracts pp. 579, 799, 803 and 2087).
The SENSE method is preferred for acceleration of the signal acquisition for magnetic resonance imaging resulting in an enormous reduction in operating time. However, the method can only be used properly if the coil sensitivity is exactly known. Otherwise imperfections will cause fold-over artefacts (aliasing) which lead to incorrect images. In practice the coil sensitivity cannot be estimated perfectly and will be dependent on fluctuations in time (movement of the patient, temperature influences, etc.).
Another important problem of the SENSE method is the spatially varying noise level in the resultant image. More specifically, the resultant image can have regions of extremely high noise level that are due to local xe2x80x9cunderdeterminationxe2x80x9d of the information provided by the coil patterns.
It is an object of the present invention to achieve a major reduction of the noise level across the entire image during fast dynamic imaging.
This and other objects of the invention are achieved by a method as defined in claim 1, by an apparatus as defined in claim 17 and by a computer program product as defined in claim 18.
The main aspect of the present invention is based on the idea to acquire the low spatial frequencies, i.e. the low-order k-lines, with full density by using the normal magnetic resonance imaging, and to acquire the remaining high spatial frequencies, i.e. the high-order k-lines, with reduced density, i.e. with the SENSE imaging method. In more general terms: it is proposed to have a k-space coverage that varies across k-space, the density of the acquired information at the centre being higher than for the high spatial frequency information.
An important additional advantage of the imaging with full density at the centre of k-space is that the SENSE acquisition can be reconstructed without a-priori knowledge of coil sensitivity maps. More accurate images with an improved signal-to-noise ratio can thus be made at the same rate as with the SENSE method.